In gyro-optical systems and other telescope applications, it is frequently desirable to scan the field of the optical objective across a radiation sensor to determine the orientation of the radiation source relative to the axis of the optical system. Of the various scanning patterns available, the rosette scan is particularly advantageous in that the relatively small field of view of a radiation sensor can be scanned across the entire field of the optical objective to produce a circular search pattern with a relatively large field of view.
The rosette pattern may be developed by combining two cone shaped scan patterns. There are a number of principal disadvantages to prior art methods of rosette scan generation. Some systems create distortion in the output signal due to chromatic and spherical aberation. Others tend to be mechanically impractical for fabrication on a production scale. Because the rosette pattern is the result of the addition of two concentric co-rotating or counter-rotating vectors, the magnitude of each must be established accurately by response of the separate optical component. A slight deflection error may produce an overlapped or incomplete closure of the center of the scan pattern. These effects are generally unacceptable for optimum operation of the system. The use of a refractive element for production of one of the scanning components may result in chromatic or other aberations that are largely uncorrectable. Furthermore the use of refractive elements to produce a scanning pattern tends to considerably restrict the wavelength regions over which the objective system must operate. Also, it is not economically feasible to cure these problems by resort to exotic or expensive materials. The optical design may be unnecessarily complicated when both refractive and reflective elements are involved in the process of generating the rosette scanning pattern.
There has, therefore, been a need for an optical scanning system that is relatively simple in construction, produces a predictable scanning pattern without error, is light in weight, and facilitates the reduction of data relative to the orientation of the detected target.